edit 2015 This question and its answers are no longer relevant. It was asked before the advent of C# 6, which has the null propagating opertor (?.)
In case you're dealing with C# 6.0/VS 2015 and above, they now have a built-in solution for null propagation:
string ans = nullableString?.Length.ToString(); // null if nullableString == null, otherwise the number of characters as a string.
The IfNotNull solution is the best (until the C# team gives us a null-safe dereferencing operator, that is).